As a biological material, wood is subject to attack by fungi and insects. These organisms may damage the appearance of the wood, and they may seriously reduce its structural strength. Wood and wood based products can be protected from wood destroying organisms by applying fungicides or insecticides, or both. Such treatments can greatly improve the service life of the wood product, especially for timbers with low natural durability, such as radiata pine.
In recent times there has been increased scrutiny of the emission of preservatives, not only during the treatment process, but from the wood after treatment.
For instance concern has been expressed with regard to the toxicity of residual arsenic in Copper Chrome Arsenic (CCA) treated wood. CCA has a long history as a low cost and effective wood preservative, but is now being excluded from certain applications where human exposure to the wood in service is high, such as playground equipment and decking. Inorganic preservatives also face the disadvantage of residual toxicity from waste. Incineration of wood waste creates volatile emissions of the heavy metals. The remaining ash will also contain toxic metal residues. Heavy metals from wood waste such as sawdust and offcuts persist in landfill and potentially pose a risk of contaminating water supplies.
Alternative, environmentally more benign, organic preservatives still have issues with regard to their application as wood preservatives.
CCA has a proven history of preserving wood over a broad range of environments, with an expectation of a service life of 25 to more than 50 years. Organic preservatives, being based on carbon chemistry, have a less certain durability. Service life must be extrapolated from field experiments typically lasting from five to ten years.
Over a time frame of 25 years or more there are additional mechanisms which might diminish the effectiveness of organic preservatives.
For instance, azoles have a limited number cell activity sites which could eventually lead to resistance and therefore might be better combined with other biocides such as the heavy metal copper. Also since they have weak bacterial activity they could eventually be subject to bacterial biodeterioration. Whereas broader spectrum biocides, such as 2-(thiocyanomethylthio)benzothiazole (TCMTB) and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (Kathon 925), which have found utility as in wood treatments, are skin irritants.
Similarly Lambda-cyhalothrin cypermethrin, deltamethrin, and bifenthrin all have a degree of paraesthesia and toxicity which makes them difficult to use in wood treatment processes.
Chlorpyrifos has appreciable vapour pressure and malodour at ambient temperatures—losses will occur through evaporation.
lodocarb, a broad spectrum biocide, hydrolyses in the presence of light and moisture over months rather than years.
A further issue with many of the organic biocides currently in use is that while they are sparingly water soluble, even this limited solubility is sometimes insufficient to resist leaching over a protracted period.
While light organic solvent carriers can be used to allow impregnation of the wood with preservative, they are undesirable as they increase expense and raise other issues of volatile organic hydrocarbon emissions into the environment. Similarly, solubilisation with surfactants to form an aqueous micro-emulsion of the active allows the use of lower cost water as a carrier, but increases the propensity of the active to leach.
Conventional methods of applying preservative treatment require a pressure difference to create an influx of fluid into the wood structure. Freshly felled wet wood freely conducts aqueous solutions. Preservative solutions may hydraulically displace the sap. More reliable treatments are achieved by first drying the timber and at least temporarily filling the empty wood cells with preservative solution. During initial drying, modification of the wood structure occurs reducing the ability of aqueous solutions to penetrate.
The most important modification from a treatment perspective is the aspiration of compound pits where the suspended torus seals up the pit opening leading to a reduction in permeability. However certain soft wood species, notably pinus radiata, can be satisfactorily treated after drying. Aqueous solutions of preservatives are pumped into the dry wood at pressures of up to 1400 Kpa which is sufficient to reopen the interconnections between the wood cells.
Pit size of softwood species is highly variable. Tracheids, one of the main types of softwood wood cell, vary in size up to a few millimeters in length, and are in diameter about 60 μm.
In green timber pit diameters vary from between 2 μm to 10 μm depending on the size of the tracheid. It is assumed that the torus in the centre of the pit which collapses across the pit opening during drying is dislodged during pressure treatment. To satisfactorily penetrate “treatable” softwood timbers it has been assumed that particulate actives must have diameters substantially less than 10 μm.
It is an object of the present invention to overcome one or more of the various disadvantages of the prior art as discussed, or at least to provide the public with a useful choice.